Various attempts have been made to use perovskite catalyst materials for the purification of exhaust gases from internal combustion engines such as automobile engines. The following references describe such materials: U.S. Pat. Nos. 3,900,428, 3,905,918, 4,049,583, 4,110,254, 4,134,852, 4,151,123 and 4,182,694, Japanese Kokai Nos. 61-082843, 61-097032, 61-149245, 62-053737, 62-065746 and 63-158130 and German Reference No. DE 3,504,556. The ability of such materials to effectively treat internal combustion exhaust gases depends on the three-way activity of the individual catalyst materials, i.e., the carbon monoxide oxidation, unsaturated hydrocarbon oxidation and nitrogen oxide reduction capabilities of the materials. Also important are the thermal stability and resistance to sulfur oxide poisoning of such materials when used as catalysts. The various perovskite catalyst materials disclosed in the aforementioned references exhibit varying degrees of three-way activity, thermal stability and sulfur oxide poisoning resistance.
It has also been suggested to use ruthenium as a catalyst component for carbon monoxide oxidation, unsaturated hydrocarbon oxidation and nitrogen oxide reduction owing to its three-way activity as set forth, for example, in U.S. Pat. Nos. 3,819,536, 3,835,069, 3,897,367, 3,900,428, 3,901,828, 3,905,918, 3,907,968, 4,001,143, 4,049,583, 4,110,254, 4,126,580, 4,134,852, 4,151,123 and 4,182,694. It has been shown that the addition of ruthenium to metallic platinum, palladium and rhodium catalysts on a support increases the selectivity of NO.sub.x reduction to nitrogen in richer mixtures and shifts the reaction of nitrogen oxides reduction to the region of lean mixtures. However, a disadvantage in the use of deposited ruthenium in such systems is the transformation of ruthenium, in the presence of oxygen, into toxic volatile RuO.sub.3 and RuO.sub.4 oxides.
Accordingly, a need exists for improved catalyst materials for use in treating internal combustion engine exhaust gases and industrial waste gases, which catalysts exhibit three-way activity for removal of carbon monoxide, unsaturated hydrocarbons and nitrogen oxides, good thermal stability and resistance to sulfur oxide poisoning.